Carbon nanotubes (abbreviated as “CNTs”) are allotropes of carbon with a cylindrical nanostructure. CNTs have been made with a length-to-diameter ratio of up to 132,000,000:1—a significantly larger ratio than for any other material. Cylindrical carbon nanotubes are known to have unusual properties which are valuable for electronics, optics, nanotechnology in general and other fields, such as materials science. CNTs have several applications in the oilfield, as well as in other industries such as the military, aerospace, and energy. Due to their extraordinary thermal conductivity and mechanical and electrical properties, CNTs find applications as additives to various structural materials. In non-limiting embodiments, nanotubes form a very small portion of the material(s) in some carbon fiber golf clubs, baseball bats, car parts and steel.
Techniques developed for producing carbon nanotubes in sizable quantities include arc discharge, laser ablation, high-pressure carbon monoxide disproportionation and chemical vapor deposition (CVD). Most of these processes require a vacuum or involve process gases. CVD growth of CNTs can occur in vacuum or at atmospheric pressure.
Arc discharge produces CNTs in the carbon soot of graphite electrodes, such as by using a current of 100 amps. The carbon in the negative electrode sublimates because of the high-discharge temperatures. The yield for this method may be up to 30 wt %, and it produces both single- and multi-walled nanotubes with lengths of up to 50 micrometers with few structural defects.
In laser ablation, a pulsed laser vaporizes a graphite target in a high-temperature reactor while an inert gas is bled into the chamber. Nanotubes develop on the cooler surfaces of the reactor as the vaporized carbon condenses. Water-cooled surfaces may be included in the system to collect the nanotubes. Good yields have been obtained by aiming the laser at composite and metal catalyst particles, such as cobalt and nickel mixture, to synthesize single-walled carbon nanotubes. The laser ablation method may produce yields around 70% and produce single-walled carbon nanotubes with a controllable diameter determined by the reaction temperature. However, this method is more expensive than either arc discharge or CVD.
Single-walled carbon nanotubes may also be synthesized by thermal plasma methods. The goal is to reproduce the conditions prevailing in the arc discharge and laser ablation methods, but a carbon-containing gas is used instead of graphite vapors to supply the carbon necessary for the production of single-walled nanotubes (SWNTs). It is continuous and relatively low cost. In a continuous process, a gas mixture composed of argon, ethylene, and ferrocene is introduced into a microwave plasma torch, where it is atomized by the atmospheric pressure plasma, which has the form of an intense “flame”. The fumes created by the flame contain SWNTs, metallic and carbon nanoparticles and amorphous carbon. The induction thermal plasma method can produce up to 2 grams of nanotube material per minute, which is a higher production rate than the arc-discharge or the laser ablation methods.
During CVD production of CNTs, a substrate is prepared with a layer of metal catalyst particles, commonly nickel, cobalt, iron or a combination of these. The metal nanoparticles may also be produced by other ways, including the reduction of oxides or oxide solid solutions. The diameters of the nanotubes that are grown are related to the size of the metal particles. This may be controlled by patterned (or masked) deposition of the metal, annealing, or by plasma etching of a metal layer. The substrate is heated to approximately 700° C. To initiate growth of nanotubes, two gases are bled into the reactor: a process gas, such as ammonia, nitrogen or hydrogen, and a carbon-containing gas, such as acetylene, ethylene, ethanol or methane. The nanotubes grow at the sites of the metal catalyst, the carbon-containing gas is broken apart at the surface of the catalyst particle, and the carbon is transported to the edges of the particle, where it forms the nanotubes. CVD is the most widely used method for the production of carbon nanotubes and may show the promise for industrial-scale production.
S. Steiner III, et al. in “Nanoscale Zirconia as a Nonmetallic Catalyst for Graphitization of Carbon and Growth of Single- and Multiwall Carbon Nanotubes,” J. Am. Chem. Soc., 2009, Vol. 131 (34), pp. 12144-12154 report that nanoparticulate zirconia (ZrO2) catalyzes both growth of single-wall and multiwall carbon nanotubes (CNTs) by thermal chemical vapor deposition (CVD) and graphitization of solid amorphous carbon. They observed that silica-, silicon nitride-, and alumina-supported zirconia on silicon nucleates single- and multiwall carbon nanotubes upon exposure to hydrocarbons at moderate temperatures (750° C.).
However, it would be desirable if a method were discovered to produce carbon nanotubes that is simpler and less expensive than current methods.